One of known devices for detecting the power level of a microwave transmitted in the waveguide is a directional coupler. The directional coupler individually detects a travelling wave and a reflected wave, which are bidirectionally transmitted in a waveguide. Directional couplers of different detection methods have been proposed. For example, various detection methods that have been proposed and actually used are: a method of transmitting a detected signal to another waveguide, a method of transmitting a detected signal to a coaxial line, and a method of transmitting a detected signal to a microstrip line.
Examples of a directional coupler that transmits a detected signal to another waveguide include a cross-shaped directional coupler described in Non-patent Document 1. In the cross-shaped directional coupler, wide faces of two waveguides are stacked into a cross shape, and connection faces of the two waveguides have two X-shaped openings at a predetermined interval.
Examples of a directional coupler that transmits a detected signal to a coaxial line include a directional coupler described in Patent Document 1. The directional coupler has an opening provided at a position corresponding to a tube axis of a wide face of a waveguide, a capacitor plate that is a microwave detecting part provided as opposed to the opening, and a detecting seat, two central conductors, and two connectors around the capacitor board.
Examples of a directional coupler that transmits a detected signal to a microstrip line include a directional coupler described in Patent Document 2. The directional coupler has an opening provided at a position corresponding to a tube axis of a wide face of a waveguide, a printed circuit board opposed to the opening, and a microstrip line that is a microwave detecting part and a detection circuit on the printed circuit board.
Examples of the directional coupler that transmits a detected signal to the microstrip line also include a directional coupler described in Patent Document 3. The directional coupler has two openings provided at positions corresponding to a tube axis of a wide face of a waveguide at a predetermined interval, a printed circuit board opposed to the two opening, and a microstrip line that is a microwave detecting part and two probes on the printed circuit board.
Although the directional couplers to be attached to the waveguide have been described, a directional coupler to be attached to a microwave heater has also been proposed (for example, refer to Patent Document 4).